Light emitting elements having several luminous sources of different colours are already known. The French patent document No. FR-A-2 186 624 provides a set of three luminous sources of different colours combined with a reflector to assure mixing of the colours. In order to individually vary the luminous intensity of each of these sources, there is employed a strip of paper provided with three tracks the transparency of which varies as a function of the illumination to be furnished at each instant by each luminous source. The transparency is measured by photo-electric cells in a manner such that when the strip moves the resulting colour gradation emitted by the reflector varies. U.S. Pat. No. 3,364,332 describes a system similar to that which has just been mentioned in which the controlling element for changing the colour gradation is in the form of a disc turning in a continuous manner.
Furthermore, it has already been suggested to control the intensity of light from a luminous source by means of a musical signal derived from a microphone or a recording placed on a support, e.g. a magnetic tape or a record. U.S. Pat. No. 3,222,574 describes such a system in which the musical signal is initially divided into three frequency bands and where the signals thus filtered are each applied to a separate lamp of which the first reacts to high frequencies, the second to medium frequencies and the third to low frequencies. These systems are presently used in recreational electronics and applied at home or in discoth/e/ ques.
Arrangements based on frequency discrimination generally also cause the luminous intensity to depend generally from the sonic volume. They exhibit however several difficulties. Initially, if one is concerned with a sound source having a limited pass band (for instance radio using amplitude modulation) the corresponding luminous gradation will exhibit a dominant colour imposed by the filter systems. If red is chosen for low frequency, green for medium frequency and blue for high frequencies, the colour gradation given by the sound of the radio in AM is located almost entirely in the red and the green as well as the mixture of these two colours. In the same manner, a rhythmic recording of which the cadence is given by contrabass chords produces almost the same effect. In these cases, the blue will be almost totally absent from the light palette. One might also cite examples in which the sonic register is carried towards high frequencies or extremely high frequencies in which case it would be the red which would appear rarely or never. Finally, it is necessary to indicate that all the systems which are proposed today exhibit a luminous intensity which varies as a function of the volume of the sound. This provokes the difficulty of having to proceed with an adjustment of the sensitivity when one goes from one source of sound (lightly recorded) to another (heavily recorded). Finally, the systems proposed show during musical silences or during soft passages of the music, undesirable "black" states.